1. Field of the Invention
The present invention relates to a viewfinder device. More particularly, the present invention relates to a viewfinder device in which a movable lens is moved in response to a zooming operation of a camera incorporating the viewfinder device.
2. Description Related to the Prior Art
A zoom viewfinder device is known in the art of the camera, and is structured to change its magnification in response to zooming of a taking lens of a camera in which the viewfinder is incorporated. The zoom viewfinder device includes a viewfinder optical system and a linking mechanism. The viewfinder optical system has a construction for the zooming or changing the magnification. The linking mechanism moves a movable lens for zooming in an optical axis direction within the viewfinder optical system in response to the zooming of the camera. The viewfinder optical system suitable for the zooming is a real image type in general. At least two movable lenses, which are disposed nearer to a photographic field than a focal plane, are moved for zooming.
In a zoom viewfinder with two movable lenses for the zooming, the movable lenses are moved in the optical axis direction with changes in an interval therebetween in response to the zooming of the camera. A lens holder is disposed to extend from the periphery of the movable lens. In other words, the lens holder holds the movable lens. A shaft receiving portion is formed with the lens holder. A guiding shaft extends in parallel with the optical axis direction, and is inserted in the shaft receiving portion, to keep the movable lens movable. Cam pins are formed with the lens holder. Two cam grooves are formed in a cam member or cam plate, and are engaged with respectively the cam pins. In the course of the zooming, the cam member is shifted in one of the two directions. The movable lenses are respectively moved by following curved shapes of the cam grooves, to change a magnification of the viewfinder continuously.
A follower projection projects from the lens holder. A guiding groove is formed in a body of the viewfinder, and extends in the optical axis direction. The follower projection is engaged with the guiding groove to keep the movable lens from moving minutely with respect to the guiding shaft. The guiding groove has a width greater than a width of the follower projection. In the prior art, a spring is used to bias the lens holder for the one-side positioning, so as to push the follower projection to one edge of the guiding groove, namely a guide face included in the guiding groove.
Ideas of biasing the lens holder are disclosed in various prior documents. In JP-A 4-159523, two lens holders are biased by respectively springs for the one-side positioning. In JP-A 6-250263, a single coil spring is disposed to receive insertion of the guiding shaft therein. The coil spring biases two lens holder portions for the one-side positioning. In both structures, the follower projection is kept in contact with the guide face. The movable lens can be moved in the optical axis direction without influence of play or backlash. Images in the view field of the zoom viewfinder can be kept stationary.
However, a problem arises in that bias force of the springs that are the coil springs or plate springs is likely to become load against movement of the lens holder. In JP-A 4-159523, the follower projections of the plural lens holders are pressed to the guide faces in a manner individual from one another. The influence of the bias force of the springs, therefore, is more remarkable. The number of the parts increases, to complicate the operation of assembly of the viewfinder. Also in JP-A 6-250263, portions of the coil spring must be engaged with a lens holder portion after the insertion of the coil spring in the guiding shaft. Again, this is inconsistent to good efficiency in the assembly operation.